Fix duplicates in thread composer

[gaearon]

Due to multiple subscriptions to the same global emitter, things were getting duplicated on the web.

Now there is only one active subscription, corresponding to the active input.

Test Plan

• Draft a thread and press Cmd+Enter.
  • Before: Thread gets posted N times.
  • After: Thread gets posted once.
• Drag an image from desktop.
  • Before: Image gets inserted into each post N times.
  • After: Image gets inserted into the current post once.
• Paste an image from clipboard.
  • Before: Image gets pasted into each post.
  • After: Image gets pasted into the current post.
• Choose an emoji in the web picker.
  • Before: Emoji gets added to every post.
  • After: Emoji gets added to the current post.

[github-actions]

Old size	New size	Diff
8.14 MB	8.14 MB	73 B (0.00%)

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[mozzius]

tested, works, lgtm!